1. Field of the Invention
This invention relates to a sizing mill used in order to roll further a metallic material in the form of a round bar, which has been subjected to coarse and intermediate rollings in a hot rolling line, into a finished product of a prescribed diameter.
2. Description of the Prior art
A sizing mill comprises two or three roll stands arranged along a planned pass line of a roll material. Each roll stand is provided with a pair of rolls formed on the outer circumferential surface thereof with respective grooves. The roll material transferred along the planned pass line is passed through the grooves on the rolls in each roll stand. In this manner, the roll material is rolled into a finished material of a prescribed diameter.
In order to raise the accuracy in the diameter of the finished material, the followings are practiced in the company where the inventor is employed. Namely, after the diameter of the finished material is decided, the diameter of the roll material is determined, taking into account of the rate of area reduction. In the next place, the radius of circular arc in cross section of the bottom surface of the above mentioned groove is determined and the depth of the groove is determined at the same time.
Accordingly, the dimension of the finished material, after being rolled with use of the rolls provided with the grooves specified in the above mentioned manner, lies within regular tolerances if the radius of the roll material is within a prescribed tolerance. However, if there is a deviation beyond the prescribed tolerance in the diameter of the roll material, there appears a problem that a deviation corresponding to this deviation in the diameter of the roll material is brought about also in the diameter of the finished material and this deviation goes beyond an allowable limit.
Furthermore, in the sizing mill including rolls provided with grooves formed in the above mentioned manner, it is necessary to change the dimensions of the groove when a finished material of a different diameter is required. In this case, it is difficult to meet this requirement only by changing the spacing between a pair of rolls. Namely, an angle .alpha. made by a line passing the center 140c and one end 140a of such portion 140 of a groove 128 that is a circular arc in cross section and a line passing the center 140c and the other end 140b of the portion 140 as shown in FIG. 15 is set equal to a large value such as 170.degree..Then the contour defined by the grooves becomes practically a round. In order to obtain a finished material, for example, of a larger diameter with use of these rolls, the spacing between the bottoms of grooves is extended from W1 to W2. Then, the shoulder dimension between a pair of rolls (the distance between the tangent at one end of the circular arc of the groove on one of the rolls and the tangent at the other end of the circular arc of the groove on the other roll) is increased from X1 to X2. The margin length X resulting from the increased shoulder dimension is very small as is shown. Accordingly, when a roll material is transferred to the rolls with the enlarged spacing between bottom surfaces W2, the cross section of the finished material passed between the rolls becomes an ellipse. Therefore, it is difficult to obtain finished materials different in diameter only by changing the spacing between a pair of rolls.
For this reason, it is necessary to change the dimensions of the groove as well in order to obtain finished materials slightly different in diameter. Moreover, it is necessary to change also the diameter of the roll material in accordance the change of the above mentioned dimensions. These changes require the work of cutting the grooves over again and the work of rearranging the rolling processes at stages before the sizing mill. These works require long time and high cost.